1. Field of the Invention
The present invention relates to an accumulation type fuel injector for use with an internal combustion engine.
2. Description of the Prior Art
The so-called accumulation type fuel injector (referred to herein as "the injector") is intended to shorten the fuel injection period thereby to complete combustion in a short time in a so-called "sharp cut fuel injection pattern", according to which the fuel injection rate is increased to a maximum and then is cut-off abruptly, so that the isochore may be enhanced to improve the thermal efficiency of the engine. This produces low smoke emission and limits the emission of nitrogen oxides even if the injection timing is retarded.
In Conventional accumulation type fuel injectors however, the needle valve for opening and closing an injection port is so designed that the effective area thereof subject to the opening pressure is larger than that subject to the closing pressure. The valve closing pressure is lower than the valve opening pressure, so that the minimum fuel injection is determined by the difference between the two pressures. Consequently the ratio of the maximum to minimum fuel injection cannot be increased and it is substantially impossible to run the internal combustion engine at low load or no load. The prior art automatic needle valve closes in response to a drop in fuel pressure within an accumulation chamber. The effective area of the head of the needle valve is reduced when the needle valve is closed, because the head of the needle valve is seated in its valve seat. The closing of the prior art needle valve is caused by its spring which carries out the work of "(the area of its sliding portion).times.(its travel).times.(the pressure applied thereto)". As a result, the needle valve closes slowly in response to the difference between its opening and closing pressures not only to make the fuel injection control difficult but also to extend the injection period so that it becomes incompatible with the aforementioned objective of reducing the injection period of the accumulation type fuel injector.
My U.S. patent application Ser. No. 570,911 filed on Jan. 16, 1984 discloses an accumulation type fuel injector of the type in which a check valve is interposed between the accumulation chamber and a fuel conduit, and an injection termination control valve is disposed in a fuel passage extending between a fuel conduit communicating with the atmospheric side of the needle valve and the accumulation chamber so that a pressure is applied to the needle valve in order to increase the closing rate thereof and thereby shorten the injection period as compared with conventional accumulation type fuel injectors.
The inventor also has proposed an accumulation type fuel injector having a needle valve control piston which has an area larger than that of the sliding portion of the needle valve. The needle valve control piston is disposed on the atmospheric side of the needle valve and a throttle is interposed between the injection termination control valve and high-pressure fuel conduit so that the needle valve opens more slowly but is closed faster due to the high pressure of fuel in the accumulation chamber acting on the control piston thereby to further shorten the fuel injection period.